1. Field of the Invention
The present invention relates to an evaporated fuel treatment device of an engine.
2. Description of the Related Art
Known in the art is an internal combustion engine provided with a canister for temporarily storing evaporated fuel, a purge control valve for controlling the amount of purge of the fuel vapor to be purged from the canister to the inside of an intake passage, and an air-fuel ratio sensor for detecting an air-fuel ratio, calculating a purge vapor concentration based on the amount of fluctuation of the air-fuel ratio, and correcting an amount of supplied fuel by the calculated purge vapor concentration so that the air-fuel ratio is maintained at a target air-fuel ratio. (see Japanese Unexamined Patent Publication (Kokai) No. 5-52139). In this internal combustion engine, so long as the purge vapor concentration is calculated correctly, the air-fuel ratio can be maintained at the target air-fuel ratio regardless of the operating state of the engine even if a purge action of the fuel vapor is performed.
Sometimes, however, the purge vapor concentration will change by a large margin if the engine operating state changes in the middle of engine operation. For example, at the time of deceleration, the purge action is normally suspended. If a large amount of fuel vapor is adsorbed by the activated carbon in the canister during this time, however, the purge vapor concentration will increase by a large margin when the purge action is restarted.
If the purge vapor concentration increases by a large margin in this way, however, the air-fuel ratio will become rich. If the air-fuel ratio becomes rich, the purge vapor concentration will start to be calculated based on the amount of fluctuation of the air-fuel ratio, but it will take time until the purge vapor concentration is accurately calculated. Therefore, for a while after the purge vapor concentration increases by a large margin, the air-fuel ratio will end up deviating to the rich side with respect to the target air-fuel ratio.
When the air-fuel ratio deviates to the rich side of the target air-fuel ratio in this way, however, if the opening degree of the purge control valve is increased rapidly, the amount of purge of a high concentration fuel vapor will be rapidly increased and therefore the air-fuel ratio will shift further to the rich side. Therefore, the air-fuel ratio will fluctuate widely.
On the other hand, part of the evaporated fuel occurring in the fuel tank is adsorbed by the activated carbon in the canister, while the remaining evaporated fuel is directly fed into the engine intake passage. In this case, the evaporated fuel fed from the fuel tank directly into the engine intake passage will depend not on the magnitude of the negative pressure occurring in the intake passage, but will depend on the amount of the evaporated fuel occurring in the fuel tank. Therefore, if the amount of intake air changes, for example, if the amount of intake air increases, the amount of purge per unit amount of intake air will decrease, so the purge vapor concentration will decrease by a large margin. As a result, the air-fuel ratio will end up deviating to the lean side of the target air-fuel ratio.
When the air-fuel ratio deviates to the lean side of the target air-fuel ratio in this way, however, if the opening degree of the purge control valve is rapidly increased, the amount of purge of the low concentration fuel vapor will be rapidly increased, so the air-fuel ratio will deviate even further to the lean side and therefore the air-fuel ratio will fluctuate by a larger margin.
In this way, if the opening degree of the purge control valve is rapidly increased when the air-fuel ratio deviates from the target air-fuel ratio, the air-fuel ratio will fluctuate by a large margin.